Themes: Add system font families missing from theme

A theme's fonts may not include all the families defined in the
system's system_fonts.xml.  If a system font family is missing
from the theme's fonts, we add it.

This works for missing monospace fonts as well as other families,
such as serif.

Change-Id: Ib2a71fab582cfdf54f4d374d0c32cfebfaa1564f
(cherry picked from commit dba4c9600ab24868c0fc2631aa0017d1d5c68737)
Theme fallback font derp fix.

Change-Id: I1c5134ca6cc3244ad409dc5450f3f10f18ca18c5
(cherry picked from commit 7340b98db8d16e56a49c41ae7f33909ef20fe97b)